1) In the Proceedings of the National Academy of Sciences (PNAS) paper "Response of Colorado River runoff to dust radiative forcing in snow" you and your team are evaluating the deserts, mountains, and runoff of the Colorado River Basin. What is the current situation?
We know that dust increases the sunlight that snow absorbs – we can see the majority of the impact with our own eyes, because the snow is darkened by the dust. This just happens to coincide with about ½ of the total blasting sunlight that reaches the snow surface. In some years, the snow is so dirty that you can barely tell that snow is peaking through. The reflectivity of relatively clean snow is somewhere between 70% and 85%. In some years, dust drives the reflectivity down to 30-40% - that means that the absorption of sunlight can more than double. To make more sense of that, think of what would happen to each of us and all of the vegetation if the Sun's input at the surface doubled – another way of thinking of it is to put all of Earth's snow closer to the Sun than Venus is (where the temperature is more than 850 °F!). Bye bye snow!
In the mountains of the Colorado River Basin, we have been measuring dust's impact on snowmelt since 2003 with high-tech weather stations. With the detailed sunlight measurements on the towers, we have been able to determine that dust itself causes snow cover to melt away by about a month earlier. In big dust years though, the melt out comes more like 50 days earlier. That's a loss of nearly two months' snow cover due to the additional sunlight going into the snowpack due to dust. Think about this – two cubic meters of snow at the time of melt weighs about a ton. Now think about the Rocky Mountains and removing a two meter snow pack one to two months earlier – think about how many tons of water is converted from snow into liquid water and rushes down into the mighty Colorado River. I still have trouble getting my head around it.
The final piece we needed before we could start figuring out how the Colorado River has changed is to know if the dust we observe now is just what it has always been or if there has been a change. A colleague of ours took his team of scientists into the high mountains but in summer when the lakes (we call many of them tarns) have melted. By coring the mud at the bottom of the lake and analyzing every which way, he and his team determined that the load of dust that we see now is about 5 times more than it was before the mid 1800s. It was around that time that railroads brought cattle and sheep to the deserts of the Western US and the surfaces of the deserts were disturbed, making dust freely available to the wind to blow to the mountains in the Colorado River Basin. So, we now know that the snowpack used to be much cleaner than it is today and therefore, used to last longer, independent of climate change.
We now pieced all of this together and have found that the snow cover used to last about 3 weeks longer than it does presently. Also, because the mountain plants and trees are exposed earlier in the season, they can start to grow earlier. When plants grow, they exhale water that they use in the process of growth and that additional 3 weeks of exhalation takes water away from the runoff. In each year, the average loss of water is about twice Las Vegas' annual use of water or about 3100 Rose Bowl stadium's full of water.
2) Did you find anything surprising?
We did – now while we weren't too surprised by how much earlier snow cover was lost, we were quite surprised that the River appears to be losing runoff. I'm still surprised by how much dust can blow from the deserts to cover all of the mountains of Colorado, Wyoming, and Utah. And that the impact is like moving us closer to the Sun than Venus!
3) What is a dust-on-snow event?
These are periods during which dust is blown from distant deserts onto snow cover – snow cover that is usually in the mountains or in the polar regions. In some mountains of the US, the dust comes from the deserts of Asia – in others like the Colorado River Basin, the dust comes primarily from the deserts of the southwest US. In general, we think about dust-on-snow events in areas where there is snow that lasts for a few months on the ground.
4) Are there ways to reverse the effects?
There are indeed. Backing up a bit, we know that disturbing desert soils tends to make the loose material underneath the crust vulnerable to be blown in the wind. We also know that surfaces that are allowed to sit during well-timed rests can recover their stability considerably. The lake records show that dust loading into the mountains decreased in the 1930s and beyond after the US Congress enacted the Taylor Grazing Act, which reduced the number of grazing animals on public lands in order to preserve soils. So, in those areas that are big sources of dust, we can devote our attention to allowing the surfaces to recover by using them more selectively in time and space. However, it is easier to say than accomplish – these desert lands have users that have myriad interests and generations of use. Even though they live in the lowlands well away from the mountains, many of these people though could also benefit from the greater duration of snow cover and potential increase in runoff. So, as with so many interactions of humans with the Earth, the solutions lie in untangling a web of interests and concerns.
5) What is the next step for your research?
We have many details to figure out in the Colorado River Basin. How does the dust loading vary from year to year and what impact is climate change going to have on the dust emission, snow cover, and in turn, the combination of dust loading on snow cover? We need to refine our remote sensing from satellites to be able to give water managers the more detailed information that they need in a timely manner.
We also are expanding this work to other mountain ranges of the US and elsewhere on Earth. For example, big dust storms that also carry black carbon hit the mountains of the Hindu Kush (Afghanistan), Karakoram (Pakistan), and Himalaya (China/Tibet, India, Nepal, and Bhutan). There are many lifetimes worth of studies to perform – which is why we need more scientists, like you!
6) How did you get interested in science? Do you have any advice to young people who might want to learn more about your field?
When I was little, we were in Zermatt, Switzerland just under the Matterhorn. There we met the granddaughter of the explorer Edward Whymper, who was famed for doing the first ascent of the Matterhorn. She told us the amazing story of his ascent while pointing up to the Matterhorn – to this day (which is a long time from that day), I can still remember the interaction of the bright sunlight with the beautiful snow on the mountain. Since then, I have been fascinated with understanding what allows snow to be beautiful and what causes it to lose its sparkly nature.
First piece of advice – always ask questions and when you get an answer, make sure that it makes sense to you. Be nice, but always press to make sure you get the explanation that makes sense. Ultimately, you will find if you become a scientist that our scientific explanations are always fuzzy – some fuzzier than others. The second piece of advice – if you think about turning on the TV, instead go get some ice from the freezer and smash it up to make your own snow (that is, if you don't live where there is snow) – take it out in the sun and sprinkle different colors of spices or dirt on your snow and see what happens. This goes for everything – a little model of the Earth and the solar system exists somewhere in your home – you just have to allow your imagination to flow. The third piece of advice … always ask questions and make sure the answer makes sense to you!